There has been disclosed many technologies of a range measurement using a radio wave, that is to say a radar function. For example, a radar which uses monotonically repeated transmitted pulses or the like is well known as a ranging function. Moreover, as a new concept wireless communication technology in recent years, attention is focused on a UWB (Ultra Wide-Band) wireless system, which is an ultra wideband wireless system utilizing a band of several GHz. And then an application using the technology has been examined not only for a communication but also for a range measurement or the like.
In the UWB wireless system, since one equipment utilizes a frequency band over several GHz, it is necessary to decrease a giving of interference to existing narrowband systems such as a fixed wireless network, a global surveillance system, and the like. Regarding an index to decrease the giving of interference, a regulation for a peak value of an average power spectral density has been examined. That is to say, the regulation, or a spectrum mask, is such that a maximum value of the average power spectrum in terms of an equivalent isotropically radiated power (EIRP) measured with a 1 MHz resolution should not exceed −41 dBm/MHz within a predetermined frequency band, for example, within a frequency range of 24 GHz to 29 GHz.
In a case of transmitting pulses with a predetermined repetition period, line spectrums occur in the average power spectral density with an interval of 1/(pulse repetition period). In order to decrease the maximum value of the average power spectral density so as not to be over the above mentioned spectrum mask, it is necessary to decrease the peak value of these line spectrums. A scrambling process is effective for this purpose, in which the pulse repetition period is randomized.
Regarding the regulation for the average power spectral density, since a spectrum observation is performed for a predetermined UWB band with a 1 MHz unit, a case of the repetition period being above 1 μs (below 1 MHz) does not make any sense in decreasing the average power spectral density. Moreover, regarding the average power spectral density, an average value observed during several tens of milliseconds is considered. Thus the peak value of the line spectrum may become higher as the pulse repetition period is made shorter (or as the repetition frequency is made higher).
Meanwhile, in a case of a short-range radar (SRR) for vehicles, a lower limit is set for the pulse repetition period. That is to say, because the ranging of approximately 30 (m) is required for the short-range radar for vehicles, it is necessary to determine the pulse repetition period so as not to output two or more pulses within the back-and-forth time of a single pulse. For example, assuming a light speed as 3×108 (m/s), a transmitting interval of the pulse should not shorter than the lowest period Tmin which is defined as the following Equation 1.
                                                                        T                ⁢                                                                  ⁢                min                            =                            ⁢                              30                ⁢                                                                  ⁢                                  (                  m                  )                                ×                                  2                  /                                      (                                          3                      ×                                              10                        8                                                              )                                                                                                                          =                            ⁢                              200                ⁢                                                                  ⁢                                                                            (                      ns                      )                                        ⁢                                                                                  [                                          PRF                      ⁢                                              :                                            ⁢                                                                                          ⁢                      5                      ⁢                                                                                          ⁢                                              (                        MHz                        )                                                              ]                                    .                                                                                        (                  Equation          ⁢                                          ⁢          1                )            Here, the second term “2” in the right-hand side expresses the back-and-forth traveling.
Thus, in the short-range radar for vehicles, it is necessary to determine the pulse repetition period within a range between 200 ns and 1 μs.
In the Patent Document 1 (see below), as the UWB wireless system, the short-range radar for vehicle configured to monitor the surrounding area of the vehicle is disclosed. A technology to perform the range measurement using one short-range radar is described herein. In order to decrease the maximum value of the above mentioned average power spectral density, the Patent Document 1 proposes a method which applies a scrambling process to the pulse repetition period by a programmable jittering device. The scrambling process has an effect to decrease a peak value of a line spectrum in an average transmit power spectral density.
Moreover, the device that simultaneously realizes a ranging function and a communicating function is being developed. For example, the Patent Document 2 (see below) discloses that both functions are realized in one device. According to the Patent Document 2, in performing data communication using a UWB wireless communication device, a communication range is predetermined using the ranging function and then a transmitting output power is determined based thereon, in order to avoid an interference with other wireless communication and the like.
Furthermore, regarding a method for obtaining a high frequency signal of the wireless communication, two methods are considered. One is the method called the direct modulation method, which directly modulates a high frequency signal by the data signal. The other is called the heterodyne method, which modulates a lower frequency using the data signal and further frequency-modulates the modulated frequency so as to obtain a necessary high frequency signal. Comparing the two methods, the direct modulation method has an advantage from the viewpoint of a circuit scale, cost performance, and the like. In executing the direct modulation method, the pulse signal is up-converted to a preferable high frequency band by a high frequency carrier wave generated by a continuous-wave oscillator. Hence, an unmodulated carrier continually appears due to the local leak, which causes a problem of deterioration of an on/off ratio of the pulse. Moreover, when modulated in a wide band, the line spectrum of the local leakage signal is more intense than the modulated signal, causing another problem of incompliance with the spectrum condition of the regulation. Therefore, a burst oscillator shown in the Non-patent Document 1 (see below) has been proposed.
In the burst oscillator of the Non-patent Document 1, a high frequency oscillator is switched on during a predetermined period only, based on a trigger signal, so as to directly obtain an RF signal of short period.    Patent Document 1 Japanese Patent Application Publication No. 2005-24563    Patent Document 2 Japanese Patent Application Publication No. 2003-174368    Nonpatent Document 1 T. Teshirogi, S. Saito, M. Uchino, M. Ejima, K. Hamaguchi, H. Ogawa and R. Kohno, “A residual-carrier-free burst oscillator for automotive UWB radar applications,” Electronics Letters, Vol. 41, No. 9, pp. 535-536, April 2005.